Replaced all internal uses of Gdx.graphics.gl20() with Gdx.gl20

Currently there are two ways to access the (actually unique) instances of `GL20` and `GL30`. Via `Gdx.gl20`/`Gdx.gl30` and `Gdx.graphics.getGL20()`/`Gdx.graphics.getGL30()`.

For an upcoming PR with some benchmarking utilities I have to replace `Gdx.gl20` with something like `Gdx.gl20 = new GL20Benchmark(Gdx.gl20)` to intercept the calls and collect statistics. However two important classes (`FrameBuffer` and `ShaderProgram`) bypass this by using `Gdx.graphics.getGL20()` instead and using an "old" copy of the `GL20` instance. Thus breaking the measuring of the shader switches for example.

To not break any existing API I have replaced all internal uses of this getter with the static property of `Gdx`.